Mega Scizor can take advantage of its defensive qualities better since it has much higher bulk, especially on the physical side which more than makes up for the lack of leftovers. Also Leftovers tend to get knocked off, but Mega Scizor can switch into Knock Off users easier and check them. It just works so much better in setup + being tough to kill and is generally viewed as a direct upgrade to vanilla Scizor. Even offensive mega Scizor is better cause of higher speed and bulk and no LO recoil making up for slightly lower power.Maybe unrelated to the current meta, but why was mega scizor so popular last gen when it is doing more or less the same thing as it is without a mega stone?
While regular Scizor can run the same sets Mega Scizor can they're basically all outclassed by the Mega form. The only thing it has to distinguish itself is a mediocre Choice Band set, which really struggles in this meta, and Fightium Z set that can can surprise some checks but has really subpar power otherwise. Basically, there's generally no reason to use regular Scizor unless you really need that mega slot for something else.